This invention relates to a novel process for the production of silica aerogels by mixing and reacting a foamed mixture of an oxidated silicon compound such as sodium silicate in the form of a powder, suspension or solution in water and a salt forming compound. Gas is utilized to foam the mixture and to produce a foamed silica gel in which some of the liquid has been replaced by a gas thereby greatly reducing the shrinkage which would occur if the gel had been dried directly from a liquid. The gas greatly reduces loss of pore volume on drying thereby producing a silica aerogel with very low density which is one of the lightest coherent solids that can be made. An aerogel is a gel in which the liquid phase has been replaced by a gaseous phase in such a way as to avoid shrinkage.
Silica aerogels have been produced by many processes which are more expensive than the process of this invention. The known processes utilize an organic liquid having a lower surface tension, such as alcohols, cresol, urea, etc., to replace most of the water in the pores of the gel then heating the silica gel in an autoclave above the critical temperature of alcohol so that there was no meniscus between the liquid and gas phases, an venting the vapors. In this way, a liquid phase was removed without subjecting the gel structure to the compressive forces owing to the surface tension of the liquid-gas interface. Silica gels and silica aerogels are extensively discussed and references are given in "THE CHEMISTRY OF SILICA", by Ralph K. Iler, a Wiley-Interscience Publication, JOHN WILEY and SONS, NEW YORK. In the process of this invention a gas is utilized instead of an organic liquid to replace the water thereby reducing the compressive forces and in the process reducing the shrinkage of the pores in the silica gel. The use of a gas such as air greatly reduces the cost by eliminating the expense of the organic liquid and the need to autoclave the silica gel to remove the organic liquid and water. The process of this invention produces a silica aerogel with the structure closely resembling the structure of the wet silica gel originally formed. The density of this silica aerogel is directly proportional to the concentration of silica in the wet silica gel. Silica aerogel containing as little silica as 0.02 g cm.sup.-3, one inch layer of silica aerogel may give up to an insulation value of R-20 between window panes especially when under a pressure of about 0.1 atmosphere or thermal conductance of 0.2 at atmospheric pressure.